Gallium nitride (GaN) semiconductors are III-V nitride semiconductors represented by a general formula BzAlxGa1-x-y-zInyN1-v-wAsvPw, where 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦x+y+z≦1, 0≦v≦1, 0≦w≦1, 0≦v+w≦1 (generally denoted as BA1GaInNAsP). A light emitting diode (hereinafter “LED”) is one known semiconductor light emitting device having a semiconductor multilayer structure each layer of which is made of a GaN semiconductor material. Such an LED emits light at a wide wavelength region from 200 nm to 1700 nm (from ultra-violet to infra-red), depending on the compositional ratios noted above. Especially, LEDs emitting blue light in a shorter wavelength range than blue-green light are now coming into wide use.
Ever increasing number of LEDs emitting blue light (blue LEDs) are widely used in electronic devices typified by mobile phones, in addition to white LEDs manufactured with blue LEDs in combination with phosphors. Furthermore, vigorous researches have been underway to use white LEDs for illumination purpose in view of its longevity superior to incandescent and halogen lamps. Currently, white LEDs are promising replacements for existing illumination sources.
In order for LEDs to be useable for a general illumination purpose, it is essential that the luminous efficiency be further improved. Generally, the luminous efficiency of LED is described by the internal quantum efficiency and the external quantum efficiency. The internal quantum efficiency is the ratio between the electric current injected into an emission layer and the amount of light produced within the emission layer. The internal quantum efficiency is proportional to the ratio of radiative recombination of electrons and positive holes. On the other hand, the external quantum efficiency is the ratio between the injection current and the amount of light extracted from the LED chip. In other words, the external quantum efficiency is the product of the internal quantum efficiency and the ratio of light emitted by the emission layer to light extracted from the LED chip (light extraction efficiency).
One basic LED has a junction structure of a p-type semiconductor layer, an emission layer, and an n-type semiconductor layer laminated in the stated order. The emission layer emits light in response to a current supplied from an n-electrode and a p-electrode formed on the respective semiconductor layers. It is important that the electrode provided on a light extraction surface does not obstruct light escaping from the LED. For example, when the p-semiconductor layer constitutes the light extraction surface, it is desirable that the p-electrode is provided at a corner of the main surface of the p-semiconductor layer in a manner of occupying a smallest possible area.
In the case of GaN semiconductor materials, it is generally difficult to manufacture a p-semiconductor layer having low resistance. With the electrodes provided as above are in sufficient to uniformly supply an electric current throughout the entire emission layer. As a result, the light emission takes place in the limited regions of the emission layer, such as directly under and in the vicinity of the electrodes. To address the above problem, one conventional technique provides a layer of transparent electrode on the entire surface of the p-semiconductor layer, and then provides a p-electrode on the transparent electrode (See JP Patent Application Publication No. 2003-110138). By the presence of the transparent electrode, an electric current supplied from the p-electrode spreads throughout the p-semiconductor layer and reaches the emission layer from the entire contacting surface. As a result, the luminance efficiency improves.
In another attempt made to improve the luminous efficiency, there is disclosed a quantum well structure, i.e. an emission layer that is made as thin as the wavelength of electron wave (See JP Patent Application Publication No. 11-330552). By employing a quantum well structure, the ratio of recombination of electrons and positive holes (radiative recombination) increases, and thus the luminous efficiency further improves.
Unfortunately, however, GaN based LEDs have the following problem, although LEDs employing a quantum well structure exhibit improved luminous efficiency than that would otherwise be.
Existing GaN semiconductor materials suffer from piezoelectric effects generated under stress induced due to the property inherent in the materials. The piezoelectric effects obstruct radiative recombination of electrons and holes, thereby decreasing the internal quantum efficiency. The mechanism of decrease will be briefly described below.
A quantum well structure improves the ratio of radiative recombination within the emission layer by confinement of electrons and positive holes (i.e. carriers) with an energy barrier. The existence probability of carriers in the well layer is obtained by a wave distribution function. The spatial overlap between electrons and positive holes (the probability existence of electrons and positive holes at the same locations) is proportional to the ratio of radiative recombination.
However, the electric field created by the piezoelectric effect scatters electrons and positive holes away toward mutually opposite ends of the well layer, thereby reducing the spatial overlap between the electrons and positive holes. This spatial separation of electrons and positive holes reduces the ratio of radiative recombination, thereby decreasing the luminous efficiency.
The piezoelectric effect can be canceled by increasing the carrier density in the well so as to cause the screening effect which compensates the internal electric field. Consequently, the spatial overlap between electrons and positive holes increases, and thus the ratio of radiative recombination increases. As a result, the internal quantum efficiency improves.
The carrier density increases with the increase of current injected to the emission layer. With the increase of current, however, it is inevitable that more heat is generated to elevate the temperature of LED chip. As a result, various problems are caused, such as deterioration of property of the LED chip itself or of resin normally provided to cover the LED chip.
In view of the above problems, the present invention aims to provide a semiconductor light emitting device with improved luminous efficiency, while maintaining the injected current within a permissible range. The present invention also aims to provide a lighting module, a lighting device, a surface mounting device, and a display device all of which employs the above semiconductor light emitting device.